Touchscreens are able to detect a touch such as by a finger or stylus within an active area. Use of a touchscreen enables a user to interact with an electronic application by touching the touchscreen. A touchscreen may include user interface constructs such as different buttons, images, or other regions that can be selected, manipulated, or actuated by touch. Such constructs may be provided by an underlying display device or may be otherwise fixed on the touchscreen. Touchscreens can therefore provide an effective user interface for cell phones, GPS devices, personal digital assistants (PDAs), computers, ATM machines, appliances and other devices.
Touchscreens use various technologies to sense touch from a finger or stylus, such as resistive, capacitive, infrared, and acoustic sensors. In one type of capacitive sensor based touchscreen, a touch changes a capacitance at a node in an array of electrodes. Capacitive touchscreens may use two separate layers of transverse electrodes, drive electrodes and sense electrodes, separated by a dielectric layer. The intersections of the transverse electrodes form the nodes. Further touchscreens may use a single layer of drive and sense electrodes. Electronics may be used to drive a series of current pulses on the drive electrodes. Charge at the nodes accumulating during each pulse is then acquired, and used to determine the location of a touch or touches.
An example two layer electrode structure for a touchscreen is illustrated in prior art FIG. 1 at 100. The structure includes drive or field emitting electrodes at 105 and receive electrodes 110. Field lines are illustrated as lines, and generally extend between the emitting electrodes 105 and receive electrodes 110. A front panel 115 may be positioned over the emitting and receive electrodes, and may be touched, such as by a finger or stylus represented at 120. Field lines proximate the touch are distorted by the touch, reducing an amount of charge that may be transferred between the transmit and receive electrodes proximate the touch.
In touchscreens with multiple drive electrodes, one drive electrode at a time is driven with pulses as indicated at signal line 205, while other drive electrodes are held at ground as indicated in a prior art timing diagram in FIG. 2. Charge on one or more receive electrodes is acquired for each sequence of pulses as indicated at charge transfer signal line 210, which is a gating signal for a charge sampler. Charge is acquired during a time that pulses in line 210 overlap pulses in line 205, referred to as a dwell time. Each drive electrode indicated at X0, X1, X2, X3, X4, X5 and X6 is driven in sequence, with acquired charge measured at the receive electrodes. The acquired charges are processed to determine a touch location on the touchscreen.